Commercially available anesthesia devices make available an exactly defined anesthetic gas concentration in the breath of a patient. Prior-art anesthetic gases are, e.g., sevoflurane, desflurane, isoflurane, enflurane and halothane. Oxygen, laughing gas or compressed air are used as carrier gases. The anesthesia device is configured, as a rule, as a closed-circuit ventilation system, whose lines are closed into a closed circuit, so that the exhaled air containing anesthetic gas or vapors is again inhaled after passing through a CO2 absorber and corresponding valves. The use of a half-closed closed-circuit ventilation system is, as a rule, preferred, so that the quantity of admitted gas exceeding the effective consumption can escape via a pressure relief valve. A certain anesthetic gas concentration, which is detected and monitored by an anesthetic gas sensor, is to be set to switch off the patient's consciousness and pain sensation. Anesthetic gas is correspondingly dispensed into the closed circuit via an anesthetic evaporator.
Anesthetic gas sensors were proposed, which are based on greatly different principles of measurement. The anesthetic gas monitor according to DE 6600383 U is based, e.g., on the change in the elasticity of a silicone rubber band, caused by the interaction with the anesthetic gas. Other methods are based on spectroscopic or spectrometric methods with the use of mass spectrometers, infrared or Raman spectroscopes, photoacoustics and piezoelectric crystal systems, where infrared spectroscopy has attained special economic significance. However, the relatively high cost of the analyzers is common to the spectroscopic gas-measuring technologies.
The anesthetic gases are, as a rule, chemically inert and at least chemically inactive. Chemical sensors are less suitable for this reason. Consuming sensors have the drawback of having a short service life, especially at relatively high concentrations (e.g., in the vol. % range in which, e.g., anesthetic gases occur).
Gas sensors with receptor surfaces on which the work function of an analyte changes as a function of the analyte concentration are of interest. Chemical reactions do not usually take place in the process.
A chemical field-effective transistor is a special form of a field-effect transistor (FET), which is used as a sensor for chemicals. The design of such a sensor essentially corresponds to that of an insulated-gate field-effect transistor (IGFET), which also includes the prior-art MOSFET, in which the conductive (usually metallic) gate electrode is removed and replaced with a receptor layer. The substances to be detected can preferably be adsorbed on the receptor layer. This can result in a concentration-dependent change in the electrical potential on the boundary surface, which causes a change in the electrical conductivity of the semiconducting channel located under the insulation layer analogously to the applied potential in conventional FETs.
Depending on the functionalization of the receptor layer, such a sensor can be used to detect atoms, molecules and ions in liquids and in the gases. Such sensors for gas analysis were first presented by Lundström in the 1970s (I. Lundstrom, S. Shivaraman, C. Svensson and L. Lundkvist, A hydrogen-sensitive MOS field effect transistor, Appl. Phys. Lett. 26, 55 (1975)). A MOSFET with a “palladium gate” electrode was used to detect hydrogen in these studies. Hydrogen is absorbed atomically by the palladium receptor layer, and a dipole layer, which is at equilibrium with the gas chemisorbed on the surface, is formed in the receptor layer.
A markedly greater variety of receptors and higher accuracy is possible with sensors in which the receptor layer and the FET are separated by an air gap and a reference electrode is used, as it is disclosed in U.S. Pat. No. 4,411,741 and EP 2006668 A1. A corresponding sensor, in which the air gap and the field-effect transistor are separated from one another in space by a gas-sensitive layer, is described in DE 4333875 C2. Such sensors are called suspended-gate sensors (SGFET). The receptor layers used in the above-described SGFET consist essentially of doped metals and compounds thereof. MOFs (metal organic frameworks) are proposed as the receptor in DE 102011075396 A1. EP 2006668 A1 proposes as the coating a monomolecular layer, which contains a silane with organic radical groups.
If the SGFET gas sensor is integrated in a standard MOSFET in the CCFET (Capacitively Coupled Field Effect Transistor) variant, the principle of function is based on the measurement of the change in the work function of a specific analyte molecule. The reaction of the receptor layer is capacitively coupled via the air gap with a subjacent MOSFET, whose channel conductivity is correspondingly modulated. A CCFET gas sensor thus comprises, in principle, an active MOSFET and a passive chip, the suspended gate, which is mounted as a carrier of the receptor layer on the MOSFET. The analyte molecule in the gas to which the CCFET sensor is sensitive is determined by the selection of the receptor layer.